1. Field of the Invention
The present invention relates to an image pickup apparatus, and the invention particularly relates to an image pickup apparatus including plural analog-to-digital conversion units.
2. Description of the Related Art
An image pickup apparatus including plural analog-to-digital conversion units (hereinafter, which will be referred to as AD conversion units) configured to perform analog-to-digital conversion (hereinafter, which will be referred to as AD conversion) is proposed. Japanese Patent Laid-Open No. 2005-323331 describes a configuration in which the AD conversion unit is provided for each pixel column. In addition, a configuration in which the AD conversion unit is provided for each pixel is described in Stuart Kleinfelder, SukHwan Lim, Xinqiao Liu, and Abbas El Gamal, “A 10000 Frames/s CMOS Digital Pixel Sensor”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, DECEMBER 2001, VOL. 36, NO. 12, pp. 2049-2059. In both documents, each AD conversion unit includes a comparator configured to output a comparison result signal indicating a result of a comparison between an analog signal and a reference signal in which a potential is changed depending on time and a counter configured to count clock signals. The AD conversion unit described in each of Japanese Patent Laid-Open No. 2005-323331 and “A 10000 Frames/s CMOS Digital Pixel Sensor” obtains a counted value of the counter at a time when a signal value of the comparison result signal is changed as a digital signal where the analog signal is subjected to the AD conversion.